1. Field of the Invention
The present invention relates to a vehicle speed control device for adjusting a speed (vehicle speed) of a vehicle having electric motors serving as a power source for separately applying power to left and right wheels.
2. Description of Related Art
In a typical four-wheel vehicle, the axles of left and right wheels are mechanically connected to each other through a differential. Power obtained from a single power source (for example, an engine) is transmitted to the left and right wheels (driving wheels) through the differential. In a vehicle turning state, the action of the differential compensates the difference in the wheel speed between the wheels due to the difference in the movement locus between the left and right wheels. As a result, the occurrence of an excessive front-rear skid (a skid in the rolling direction of the vehicle) of the left and right wheels is suppressed.
In the meantime, in recent years, there has been developed a vehicle (in-wheel motor vehicle) having electric motors (called in-wheel motors) respectively provided into the tire wheels of the left and right wheels (for example, see JP-A-2007-269129 and JP-A-2008-068832). In the in-wheel motor vehicle, no differential is provided, and the output shafts of the electric motors for the left and right wheels are not mechanically connected to each other. The electric motors for the left and right wheels are separately controlled, that is, the wheel speeds, the shaft torque, and the like of the left and right wheels can be separately controlled.
The skid of the wheel includes the above-described front-rear skid and a lateral skid which is a skid in the lateral direction with respect to the wheel. If it is assumed that the steering angles of the left and right steering wheels are adjusted in faithful accordance with the theoretical characteristics of the Ackermann geometry, the steering angle (inner wheel steering angle) of a turning inside wheel becomes greater than the steering angle (outer wheel steering angle) of a turning outside wheel. Then, under the condition that the turning center is placed on the extension line of the rear wheel axle, the lateral skid of each wheel in the vehicle turning state becomes zero (in a very low-speed range where centrifugal force is negligible).
However, in practice, it is very difficult to design a steering device in which the steering angles of the left and right steering wheels are adjusted in faithful accordance with the theoretical characteristics of the Ackermann geometry. In an actual steering device, the steering angles of the left and right steering wheels are adjusted in accordance with the characteristics of steering geometry close to Ackermann geometry such that the inner wheel steering angle becomes greater than the outer wheel steering angle. Accordingly, it is possible to suppress the lateral skid of each wheel in the vehicle turning state.